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Intact male and ovariectomized female rats were trained to lever press under a random ratio (RR) schedule of food reinforcement. Effects of different doses of pentobarbital (1-16 mg/kg) and progesterone (10-80 mg/kg) on response output were studied. Low doses of pentobarbital increased responding, high doses decreased responding. Sex differences were observed in the rate-decreasing effects of high doses of pentobarbital. Progesterone increased the response rate of ovariectomized females, but did not affect responding in intact males.